Actin cytoskeletal architecture regulates nitric oxide-induced apoptosis, dedifferentiation, and cyclooxygenase-2 expression in articular chondrocytes via mitogen-activated protein kinase and protein kinase C pathways

P53: A Key Target in the Development of Osteoarthritis

Osteoarthritis (OA), a chronic degenerative disease characterized mainly by damage to the articular cartilage, is increasingly relevant to the pathological processes of senescence, apoptosis, autophagy, proliferation, and differentiation of chondrocytes. Clinical strategies for osteoarthritis can only improve symptoms and even along with side effects due to age, sex, disease, and other factors. Therefore, there is an urgent need to identify new ideas and targets for current clinical treatment. The tumor suppressor gene p53, which has been identified as a potential target for tumor therapeutic intervention, is responsible for the direct induction of the pathological processes involved in OA modulation. Consequently, deciphering the characteristics of p53 in chondrocytes is essential for investigating OA pathogenesis due to p53 regulation in an array of signaling pathways. This review highlights the effects of p53 on senescence, apoptosis, and autophagy of chondrocytes and its role in the development of OA. It also elucidates the underlying mechanism of p53 regulation in OA, which may help provide a novel strategies for the clinical treatment of OA.

Transit Amplifying Cells (TACs): a still not fully understood cell population

Maintenance of tissue homeostasis and tissue regeneration after an insult are essential functions of adult stem cells (SCs). In adult tissues, SCs proliferate at a very slow rate within "stem cell niches", but, during tissue development and regeneration, before giving rise to differentiated cells, they give rise to multipotent and highly proliferative cells, known as transit-amplifying cells (TACs). Although differences exist in diverse tissues, TACs are not only a transitory phase from SCs to post-mitotic cells, but they also actively control proliferation and number of their ancestor SCs and proliferation and differentiation of their progeny toward tissue specific functional cells. Autocrine signals and negative and positive feedback and feedforward paracrine signals play a major role in these controls. In the present review we will consider the generation and the role played by TACs during development and regeneration of lining epithelia characterized by a high turnover including epidermis and hair follicles, ocular epithelial surfaces, and intestinal mucosa. A comparison between these different tissues will be made. There are some genes and molecular pathways whose expression and activation are common to most TACs regardless their tissue of origin. These include, among others, Wnt, Notch, Hedgehog and BMP pathways. However, the response to these molecular signals can vary in TACs of different tissues. Secondly, we will consider cultured cells derived from tissues of mesodermal origin and widely adopted for cell therapy treatments. These include mesenchymal stem cells and dedifferentiated chondrocytes. The possible correlation between cell dedifferentiation and reversion to a transit amplifying cell stage will be discussed.

Global research trends and hotspots of PI3K/Akt signaling pathway in the field of osteoarthritis: A bibliometric study

The phosphatidylinositol 3-kinase/protein kinase B (PI3K/Akt) signaling pathway has gradually become a new target for the treatment of osteoarthritis (OA). Numerous studies of PI3K/Akt signaling in OA have been published in the past few years. By analyzing these research characteristics and qualities, we aimed to reveal the current research focus and emerging trends in PI3K/Akt signaling in OA. We searched the Web of Science database for relevant articles concerning the PI3K/Akt signaling pathway in OA published from inception to October 31, 2022. The following data were extracted: author name, article title, keywords, topic, publication country/region, institution, publication journal, journal impact factor, number of times cited, and H-index. VOSviewer and Excel 2019 were used to conduct the bibliometric study and visualize the analysis. A total of 374 publications were included in this study. In all selected articles, "orthopedics" was the dominant topic (252 of 374, 67.38%). The most productive year was 2021. Frontiers in Pharmacology published the most articles. The People's Republic of China has published the most articles worldwide. The top 5 keywords were "OA," "expression," "apoptosis," "chondrocytes," and "inflammation." The keywords "autophagy," "mitochondrial dysfunction," "inflammatory response," "cartilage degeneration," and "network pharmacology" have increased in recent years. Our study showed a growing trend in published articles related to the PI3K/Akt signaling pathway in OA. Inflammatory response, cartilage degeneration, and apoptosis remain central topics in the field. Research on autophagy, mitochondrial dysfunction, and network pharmacology is on the rise, and the focus on PI3K/Akt will continue to increase.

Upregulated endonuclease Regnase-1 suppresses osteoarthritis by forming a negative feedback loop of catabolic signaling in chondrocytes

BACKGROUND

Ribonucleases (RNases) play central roles in the post-transcriptional regulation of mRNA stability. Our preliminary results revealed that the endonuclease Regnase-1 is specifically upregulated in osteoarthritic chondrocytes. We herein explored the possible functions and regulatory mechanisms of Regnase-1 in a mouse model of osteoarthritis (OA).

METHODS

The expression and target genes of Regnase-1 were identified by microarray analysis in primary-culture mouse articular chondrocytes. Experimental OA in mice was induced by destabilization of the medial meniscus (DMM). The function of Regnase-1 in DMM-induced post-traumatic OA mice was examined by adenovirus-mediated overexpression or knockdown in knee joint tissues, and also by using Regnase-1 heterozygous knockout mice (Zc3h12a^+/-).

RESULTS

Among the RNases, Regnase-1 was exclusively upregulated in chondrocytes stimulated with OA-associated catabolic factors. Adenovirus-mediated overexpression or knockdown of Regnase-1 alone in joint tissues did not cause OA-like changes. However, overexpression of Regnase-1 in joint tissues significantly ameliorated DMM-induced post-traumatic OA cartilage destruction, whereas knockdown or genetic ablation of Regnase-1 exacerbated DMM-induced cartilage destruction. Mechanistic studies suggested that Regnase-1 suppresses cartilage destruction by modulating the expression of matrix-degrading enzymes in chondrocytes.

CONCLUSION

Our results collectively suggest that upregulated Regnase-1 in OA chondrocytes may function as a chondro-protective effector molecule during OA pathogenesis by forming a negative feedback loop of catabolic signals, such as matrix-degrading enzyme expression, in OA chondrocytes.

Involvement of the Actin Machinery in Programmed Cell Death

Programmed cell death (PCD) depicts a genetically encoded and an orderly mode of cellular mortality. When triggered by internal or external stimuli, cells initiate PCDs through evolutionary conserved regulatory mechanisms. Actin, as a multifunctional cytoskeleton protein that forms microfilament, its integrity and dynamics are essential for a variety of cellular processes (e.g., morphogenesis, membrane blebbing and intracellular transport). Decades of work have broadened our knowledge about different types of PCDs and their distinguished signaling pathways. However, an ever-increasing pool of evidences indicate that the delicate relationship between PCDs and the actin cytoskeleton is beginning to be elucidated. The purpose of this article is to review the current understanding of the relationships between different PCDs and the actin machinery (actin, actin-binding proteins and proteins involved in different actin signaling pathways), in the hope that this attempt can shed light on ensuing studies and the development of new therapeutic strategies.

Dedifferentiation: inspiration for devising engineering strategies for regenerative medicine

Cell dedifferentiation is the process by which cells grow reversely from a partially or terminally differentiated stage to a less differentiated stage within their own lineage. This extraordinary phenomenon, observed in many physiological processes, inspires the possibility of developing new therapeutic approaches to regenerate damaged tissue and organs. Meanwhile, studies also indicate that dedifferentiation can cause pathological changes. In this review, we compile the literature describing recent advances in research on dedifferentiation, with an emphasis on tissue-specific findings, cellular mechanisms, and potential therapeutic applications from an engineering perspective. A critical understanding of such knowledge may provide fresh insights for designing new therapeutic strategies for regenerative medicine based on the principle of cell dedifferentiation.

Identification of DNA hydroxymethylation associated genes in osteoarthritis by combined analysis of hydroxymethylation and gene expression

BACKGROUND

Our study aimed to explore more mechanistic insights into the epigenetic regulation of osteoarthritis (OA).

METHODS

The expression profiles (accession number: GSE64393 and GSE64394) were downloaded from the Gene Expression Omnibus database. The differentially hydroxymethylated regions (DhMRs) and differentially expressed genes (DEGs) between OA and control groups were identified. The distribution of DhMRs in the whole genome and the correlation between DhMRs and DEGs were analyzed. Functional module mining for the DEGs and DhMRs was conducted, followed by protein-protein interaction (PPI) analysis. The transcriptional factor (TF) was predicted.

RESULTS

Total 52,282 DhMRs were obtained, among which 31,452 ones were annotated to 9726 genes. Additionally, 1806 DEGs were selected. Hydroxymethylation mainly occurred in gene body region. Correlation analysis revealed that more than 70% of DhMRs were uncorrelated with DEGs expression. Functional module mining for the DEGs and DhMRs identified 2 functional modules, which were involved in pathways of regulation of actin cytoskeleton, and TGF-β signaling pathway. A PPI network was constructed, and ITGB3 had the highest degree. Furthermore, 7 TFs were predicted, which regulated 12 candidate genes, such as HES1-PTEN.

CONCLUSIONS

The onset and progression of OA may be associated with the upregulated hydroxymethylation in gene body region of PTEN. HES1 may be important TF in the pathogenesis of OA. Additionally, pathways of regulation of actin cytoskeleton, and TGF-beta signaling pathway may also play important roles in OA progression.

The PI3K/AKT/mTOR signaling pathway in osteoarthritis: a narrative review

Osteoarthritis (OA) is a complicated degenerative disease that affects whole joint tissue. Currently, apart from surgical approaches to treat late stage OA, effective treatments to reverse OA are not available. Thus, the mechanisms leading to OA, and more effective approaches to treat OA should be investigated. According to available evidence, the PI3K/AKT/mTOR signaling pathway is essential for normal metabolism of joint tissues, but is also involved in development of OA. To provide a wide viewpoint to roles of PI3K/AKT/mTOR signaling pathway in osteoarthritis, a comprehensive literature search was performed using PubMed terms 'PI3K OR AKT OR mTOR' and 'osteoarthritis'. This review highlights the role of PI3K/AKT/mTOR signaling in cartilage degradation, subchondral bone dysfunction, and synovial inflammation, and discusses how this signaling pathway affects development of the disease. We also summarize recent evidences of therapeutic approaches to treat OA by targeting the PI3K/AKT/mTOR pathway, and discuss potential challenges in developing these strategies for clinical treatment of OA.

Adseverin, an actin binding protein, regulates articular chondrocyte phenotype

Chondrocytes dedifferentiate as a result of monolayer culture for cell number expansion. This is associated with the development of an elongated shape, increased actin polymerization, development of stress fibres, and expression of contractile molecules. Given the changes in actin status with dedifferentiation, the hypothesis of this study was that adseverin, an actin severing and capping protein, plays a role in regulating chondrocyte phenotype and function. This study reports that serial passaging of articular chondrocytes in monolayer culture resulted in loss of adseverin protein expression as early as Day 14 of culture and remained repressed in Passage 2 (P2) cells. Knockdown of adseverin by siRNA in primary chondrocytes promoted an increase in cell size and an elongated shape, actin stress fibres, decreased G-/F-actin ratio, and increased number of actin-free barbed ends. The cells also showed increased expression of the contractile genes and proteins, vinculin and α-smooth muscle actin, and increased ability to contract collagen gels. These are all features of dedifferentiation. These effects were due to adseverin as adseverin overexpression following transfection of the green fluorescent protein-adseverin plasmid partially reversed all of these changes in P2 chondrocytes. Furthermore, sox9 and aggrecan chondrogenic gene expression was upregulated, and collagen type I genes expression was downregulated with adseverin overexpression. The change in aggrecan mRNA expression had functional consequence as these cells exhibited increased total proteoglycan synthesis. These findings demonstrate that adseverin regulates features indicative of redifferentiation in passaged articular chondrocytes through modulation of the actin cytoskeleton status and potentially may regulate the maintenance of phenotype in primary chondrocytes.

Simvastatin prevents articular chondrocyte dedifferentiation induced by nitric oxide by inhibiting the expression of matrix metalloproteinases 1 and 13

IMPACT STATEMENT

Dedifferentiation of chondrocytes is the main character of cartilage degradation. Therefore the understanding of chondrocytes dedifferentiation is essential for arthritis therapy. However, the molecular mechanism of cartilage destroy is mostly unknown. In this work we show that simvastatin (SVT) inhibits dedifferentiation by nitric oxide by blocking the expression of matrix metalloproteinases 1 and 13. These effects of SVT on dedifferentiation suggest that SVT may be used as a drug for the cure of arthritis.

Steroidal glycoalkaloids from Solanum nigrum target cytoskeletal proteins: an in silico analysis

Background

Solanum nigrum (black nightshade; S. nigrum), a member of family Solanaceae, has been endowed with a heterogeneous array of secondary metabolites of which the steroidal glycoalkaloids (SGAs) and steroidal saponins (SS) have vast potential to serve as anticancer agents. Since there has been much controversy regarding safety of use of glycoalkaloids as anticancer agents, this area has remained more or less unexplored. Cytoskeletal proteins like actin play an important role in maintaining cell shape, synchronizing cell division, cell motility, etc. and along with their accessory proteins may also serve as important therapeutic targets for potential anticancer candidates. In the present study, glycoalkaloids and saponins from S. nigrum were screened for their interaction and binding affinity to cytoskeletal proteins, using molecular docking.

Methods

Bioactivity score and Prediction of Activity Spectra for Substances (PASS) analysis were performed using softwares Molinspiration and Osiris Data Explorer respectively, to assess the feasibility of selected phytoconstituents as potential drug candidates. The results were compared with two standard reference drugs doxorubicin hydrochloride (anticancer) and tetracycline (antibiotic). Multivariate data obtained were analyzed using principal component analysis (PCA).

Results

Docking analysis revealed that the binding affinities of the phytoconstituents towards the target cytoskeletal proteins decreased in the order coronin>villin>ezrin>vimentin>gelsolin>thymosin>cofilin. Glycoalkaloid solasonine displayed the greatest binding affinity towards the target proteins followed by alpha-solanine whereas amongst the saponins, nigrumnin-I showed maximum binding affinity. PASS Analysis of the selected phytoconstituents revealed 1 to 3 violations of Lipinski’s parameters indicating the need for modification of their structure-activity relationship (SAR) for improvement of their bioactivity and bioavailability. Glycoalkaloids and saponins all had bioactivity scores between −5.0 and 0.0 with respect to various receptor proteins and target enzymes. Solanidine, solasodine and solamargine had positive values of druglikeness which indicated that these compounds have the potential for development into future anticancer drugs. Toxicity potential evaluation revealed that glycoalkaloids and saponins had no toxicity, tumorigenicity or irritant effect(s). SAR analysis revealed that the number, type and location of sugar or the substitution of hydroxyl group on alkaloid backbone had an effect on the activity and that the presence of α-L-rhamnopyranose sugar at C-2 was critical for a compound to exhibit anticancer activity.

Conclusion

The present study revealed some cytoskeletal target(s) for S. nigrum phytoconstituents by docking analysis that have not been previously reported and thus warrant further investigations both in vitro and in vivo.

Simvastatin-dependent actin cytoskeleton rearrangement regulates differentiation via the extracellular signal-regulated kinase-1/2 and p38 kinase pathways in rabbit articular chondrocytes

Alterations in cell morphology involve changes in the actin cytoskeleton and play crucial roles in determining chondrocyte phenotypes. Although the effects of simvastatin (SV) have been demonstrated in various cell types, the mechanisms and effects of SV on chondrocyte differentiation and actin cytoskeletal rearrangement are still unclear. Here, we investigated the roles of actin filament rearrangement on SV-induced differentiation of rabbit articular chondrocytes. Treatment with SV caused actin remodeling in comparison with that in untreated chondrocytes, as determined by immunofluorescence staining. Moreover, treatment with cytochalasin D (CD) and jasplakinolide (JAS), which modulate actin filament formation, resulted in reorganization of the actin cytoskeleton compared with that induced by SV in chondrocytes. In addition, CD synergistically enhanced the SV-induced increase in type II collagen expression, whereas JAS dramatically inhibited SV-induced differentiation. We also found that differentiation via SV-dependent actin cytoskeleton changes was regulated by the extracellular signal-regulated kinase (ERK)-1/2 and p38 kinase pathways. These results demonstrated that actin cytoskeletal rearrangement by SV regulated type II collagen expression and suggested that ERK-1/2 and p38 kinase pathways may play important roles in SV-induced type II collagen expression by altering actin cytoskeletal reorganization in rabbit articular chondrocytes.

Dominant roles of Fenton reaction in sodium nitroprusside-induced chondrocyte apoptosis

Sodium nitroprusside (SNP) has been widely used as an exogenous nitric oxide (NO) donor to explore the molecular mechanism of NO-mediated chondrocyte apoptosis during the latest two decades. We have recently found that NO-independent ROS play a key role in SNP-induced apoptosis in rabbit chondrocytes. This study aims to investigate what kind of ROS and how the reliable ROS mediators mediate the SNP-induced apoptosis. Data shows that SNP and NO-exhausted SNP (SNPex) induced ROS production or cytotoxicity to identically degree. SNP induced a marked increase in iron ions, superoxide anion (O2(•-)), hydrogen peroxide (H2O2) and hydroxyl radical ((•)OH) level. H2O2 scavenger (CAT) and (•)OH scavenger (DMSO) significantly inhibited SNP-induced chondrocyte apoptosis. Iron ions chelator (DFO) entirely prevented SNP-induced chondrocyte apoptosis. In contrast, O2(•-) scavenger (SOD) and glutathione depletion agent (BSO) promoted SNP-induced cytotoxicity. K3[Fe(CN)6] exhibited no cytotoxicity, and H2O2 alone up to 250µM or iron ions alone up to 90µM is non-cytotoxic to chondrocytes. Combination of 25µM FeSO4 and 100µM H2O2 in the presence of BSO induced chondrocyte death similar to SNP treatment. Fetal bovine serum (FBS) enhanced iron ions release from SNP and the cytotoxicity of SNP. Our data shows that the extracellular Fenton reaction between iron ions released from SNP and H2O2 induced by SNP plays a key role in SNP-induced chondrocyte apoptosis. Overall, our results indicate that the potential of SNP to increase iron ions and ROS should be especially considered for some biological functions and, possibly, also for clinical applications of this drug.

Berberine induces dedifferentiation by actin cytoskeleton reorganization via phosphoinositide 3-kinase/Akt and p38 kinase pathways in rabbit articular chondrocytes

Osteoarthritis is a nonrheumatologic joint disease characterized by progressive degeneration of the cartilage extracellular matrix. Berberine (BBR) is an isoquinoline alkaloid used in traditional Chinese medicine, the majority of which is extracted from Huang Lian (Coptis chinensis). Although numerous studies have revealed the anticancer activity of BBR, its effects on normal cells, such as chondrocytes, and the molecular mechanisms underlying its actions remain elusive. Therefore, we examined the effects of BBR on rabbit articular chondrocytes, and the underlying molecular mechanisms, focusing on actin cytoskeletal reorganization. BBR induced dedifferentiation by inhibiting activation of phosphoinositide-3(PI3)-kinase/Akt and p38 kinase. Furthermore, inhibition of p38 kinase and PI3-kinase/Akt with SB203580 and LY294002, respectively, accelerated the BBR-induced dedifferentiation. BBR also caused actin cytoskeletal architecture reorganization and, therefore, we investigated if these effects were involved in the dedifferentiation. Disruption of the actin cytoskeleton by cytochalasin D reversed the BBR-induced dedifferentiation by activating PI3-kinase/Akt and p38 kinase. In contrast, the induction of actin filament aggregation by jasplakinolide accelerated the BBR-induced dedifferentiation via PI3-kinase/Akt inhibition and p38 kinase activation. Taken together, these data suggest that BBR strongly induces dedifferentiation, and actin cytoskeletal reorganization is a crucial requirement for this effect. Furthermore, the dedifferentiation activity of BBR appears to be mediated via PI3-kinase/Akt and p38 kinase pathways in rabbit articular chondrocytes.

Resveratrol protects chondrocytes from apoptosis via altering the ultrastructural and biomechanical properties: an AFM study

Osteoarthritis (OA), a degenerative joint disease with high prevalence among older people, occurs from molecular or nanometer level and extends gradually to higher degrees of the ultrastructure of cartilage, finally resulting in irreversible structural and functional damages. This report aims to use atomic force microscopy (AFM) to investigate the protective effects of resveratrol (RV), a drug with good anti-inflammatory properties, on cellular morphology, membrane architecture, cytoskeleton, cell surface adhesion and stiffness at nanometer level in sodium nitroprusside (SNP)-induced apoptotic chondrocytes, a typical cellular OA model. CCK-8 assay showed that 100 μM RV significantly prevented SNP-induced cytotoxicity. AFM imaging and quantitative analysis showed that SNP potently induced chondrocytes changes including shrunk, round, lamellipodia contraction and decrease in adherent junctions among cells, as well as the destruction of biomechanics: 90% decrease in elasticity and 30% decrease in adhesion. In addition, confocal imaging analysis showed that SNP induced aggregation of the cytoskeleton and decrease in the expression of cytoskeletal proteins. More importantly, these SNP-induced damages to chondrocytes could be potently prevented by RV pretreatment. Interestingly, the biomechanical changes occurred before morphological changes could be clearly observed during SNP-induced apoptosis, indicating that the biomechanics of cellular membrane may be a more robust indicator of cell function. Collectively, our data demonstrate that RV prevents SNP-induced apoptosis of chondrocytes by regulating actin organization, and that AFM-based technology can be developed into a powerful and sensitive method to study the interaction mechanisms between chondrocytes and drugs.

Regulation of chondrogenesis by protein kinase C: Emerging new roles in calcium signalling

During chondrogenesis, complex intracellular signalling pathways regulate an intricate series of events including condensation of chondroprogenitor cells and nodule formation followed by chondrogenic differentiation. Reversible phosphorylation of key target proteins is of particular importance during this process. Among protein kinases known to be involved in these pathways, protein kinase C (PKC) subtypes play pivotal roles. However, the precise function of PKC isoenzymes during chondrogenesis and in mature articular chondrocytes is still largely unclear. In this review, we provide a historical overview of how the concept of PKC-mediated chondrogenesis has evolved, starting from the first discoveries of PKC isoform expression and activity. Signalling components upstream and downstream of PKC, leading to the stimulation of chondrogenic differentiation, are also discussed. Although it is evident that we are only at the beginning to understand what roles are assigned to PKC subtypes during chondrogenesis and how they are regulated, there are many yet unexplored aspects in this area. There is evidence that calcium signalling is a central regulator in differentiating chondroprogenitors; still, clear links between intracellular calcium signalling and prototypical calcium-dependent PKC subtypes such as PKCalpha have not been established. Exploiting putative connections and shedding more light on how exactly PKC signalling pathways influence cartilage formation should open new perspectives for a better understanding of healthy as well as pathological differentiation processes of chondrocytes, and may also lead to the development of novel therapeutic approaches.

Resveratrol Inhibits Nitric Oxide-Induced Apoptosis via the NF-Kappa B Pathway in Rabbit Articular Chondrocytes

Resveratrol (trans-3,4'-trihydroxystillbene), a naturally occurring polyphenolic antioxidant found in grapes and red wine, elicits diverse biochemical responses and demonstrates anti-aging, anti-inflammatory, and anti-proliferative effects in several cell types. Previously, resveratrol was shown to regulate differentiation and inflammation in rabbit articular chondrocytes, while the direct production of nitric oxide (NO) in these cells by treatment with the NO donor sodium nitroprusside (SNP) led to apoptosis. In this study, the effect of resveratrol on NO-induced apoptosis in rabbit articular chondrocytes was investigated. Resveratrol dramatically reduced NO-induced apoptosis in chondrocytes, as determined by phase-contrast microscopy, the MTT assay, FACS analysis, and DAPI staining. Treatment with resveratrol inhibited the SNP-induced expression of p53 and p21 and reduced the expression of procaspase-3 in chondrocytes, as detected by western blot analysis. SNP-induced degradation of I-kappa B alpha (IκB-α) was rescued by resveratrol treatment, and the SN50 peptide-mediated inhibition of NF-kappa B (NF-κB) activity potently blocked SNP-induced caspase-3 activation and apoptosis. Our results suggest that resveratrol inhibits NO-induced apoptosis through the NF-κB pathway in articular chondrocytes.

Actin cytoskeleton modulates ADMA-induced NF-kappaB nuclear translocation and ICAM-1 expression in endothelial cells

BACKGROUND

Asymmetric dimethylarginine (ADMA), an endogenous nitric oxide synthase (NOS) inhibitor, increases the activity of NF-κB (NF-κB) and then induces the expression of intercellular adhesion molecule-1 (ICAM-1). However, the mechanisms regulating ADMA-induced NF-κB activation are unknown. This study investigated the function of actin cytoskeleton for ADMA-induced NF-κB activation and ICAM-1 expression in endothelial cells.

MATERIAL/METHODS

Human umbilical vein endothelial cells (HUVEC) were cultured and left untreated or challenged for 24 h with 100 µM ADMA in the absence and presence of 5 µM cytochalasin D (Cyt D), or 1 µM Jasplakinolide (Jas). The form of actin cytoskeleton, the translocation of NF-κB, NF-κB DNA binding activity, and the expression of ICAM-1 were determined.

RESULTS

ADMA increased the formation of stress fiber in endothelial cells, and Cyt D clearly induced destabilization of the actin filaments. Either stabilizing or destabilizing the actin cytoskeleton prevented ADMA-induced NF-κB activation. It also showed that the inhibition of NF-κB activity was due to the impaired NF-κB nuclear translocation. Further, stabilizing or destabilizing the actin cytoskeleton inhibited the expression of the NF-κB target protein, ICAM-1.

CONCLUSIONS

Actin cytoskeleton may be engaged in modulated ADMA-induced NF-κB activation and thereby ICAM-1 expression in endothelial cells.

Apoptosis in the anucleate platelet

For many years, programmed cell death, known as apoptosis, was attributed exclusively to nucleated cells. Currently, however, apoptosis is also well-documented in anucleate platelets. This review describes extrinsic and intrinsic pathways of apoptosis in nucleated cells and in platelets, platelet apoptosis induced by multiple chemical stimuli and shear stresses, markers of platelet apoptosis, mitochodrial control of platelet apoptosis, and apoptosis mediated by platelet surface receptors PAR-1, GPIIbIIIa and GPIbα. In addition, this review presents data on platelet apoptosis provoked by aging of platelets in vitro during platelet storage, platelet apoptosis in pathological settings in humans and animal models, and inhibition of platelet apoptosis by cyclosporin A, intravenous immunoglobulin and GPIIbIIIa antagonist drugs.

Reactive oxygen species and nitric oxide mediate actin reorganization and programmed cell death in the self-incompatibility response of papaver

Pollen-pistil interactions are critical early events regulating pollination and fertilization. Self-incompatibility (SI) is an important mechanism to prevent self-fertilization and inbreeding in higher plants. Although data implicate the involvement of reactive oxygen species (ROS) and nitric oxide (NO) in pollen-pistil interactions and the regulation of pollen tube growth, there has been a lack of studies investigating ROS and NO signaling in pollen tubes in response to defined, physiologically relevant stimuli. We have used live-cell imaging to visualize ROS and NO in growing Papaver rhoeas pollen tubes using chloromethyl-2'7'-dichlorodihydrofluorescein diacetate acetyl ester and 4-amino-5-methylamino-2',7'-difluorofluorescein diacetate and demonstrate that SI induces relatively rapid and transient increases in ROS and NO, with each showing a distinctive "signature" within incompatible pollen tubes. Investigating how these signals integrate with the SI responses, we show that Ca(2+) increases are upstream of ROS and NO. As ROS/NO scavengers alleviated both the formation of SI-induced actin punctate foci and also the activation of a DEVDase/caspase-3-like activity, this demonstrates that ROS and NO act upstream of these key SI markers and suggests that they signal to these SI events. These data represent, to our knowledge, the first steps in understanding ROS/NO signaling triggered by this receptor-ligand interaction in pollen tubes.

Role of the cytoskeleton in Cd2+-induced death of mouse mesangial cells

Cadmium induces apoptotic cell death in mouse mesangial cells that is in part dependent on reactive oxygen species (ROS). Cadmium also activates multiple kinases in these cells, including the Ca2+/calmodulin-dependent protein kinase II (CaMK-II) and p38 kinase, and also leads to disruption of the filamentous actin cytoskeleton. We investigated the role of the cytoskeleton in Cd2+-induced cell death. Cell viability was decreased by Cd2+ and two types of apoptotic death, defined by flow cytometry, were increased. Disruption of actin filaments with cytochalasin D was partially protective, whereas stabilization of the cytoskeleton with jasplakinolide was without effect, indicating that cytoskeletal disruption contributes to, but is not necessary for, induction of apoptosis. Inhibition of CaMK-II and p38 kinase, mitochondrial stabilization with cyclosporine A, and the antioxidant N-acetyl cysteine all protected against apoptosis and prevented disruption of the cytoskeleton. Cytochalasin D decreased Cd2+-dependent ROS production, reduced the decline in mitochondrial membrane potential, and decreased phosphorylation of p38 kinase. We conclude that Cd2+-dependent actin disruption is a downstream event facilitating apoptotic death. Cadmium-dependent cell death involves actin-dependent mitochondrial changes, ROS production, and p38 activation.

Redox regulation of actin by thioredoxin-1 is mediated by the interaction of the proteins via cysteine 62

Actin is a highly conserved protein in eukaryotic cells, and has been identified as one of the main redox targets by redox proteomics under oxidative stress. However, little is known about the mechanisms of regulation of the redox state of actin. In this study, we investigated how thioredoxin-1 (Trx1) affected the redox state of actin and its polymerization under oxidative stress in SH-SY5Y cells. Trx1 decreased the levels of reactive oxygen species (ROS) in the cells, and cysteine residues at positions 32, 35, and 69 of the Trx1 protein were active sites for redox regulation. Actin could be kept in a reduced state by Trx1 under H(2)O(2) stimulation. A physical interaction was found to exist between actin and Trx1. Cysteine 62 in Trx1 was the key site that interacted with actin, and it was required to maintain cellular viability and anti-apoptotic function. Taken together, these results suggested that Trx1 could protect cells from apoptosis under oxidative stress not only by increasing the total antioxidant capability and decreasing the ROS levels, but also by stabilizing the actin cytoskeletal system, which cooperatively contributed to the enhancement of cell viability and worked against apoptosis.

Joint aging and chondrocyte cell death

Articular cartilage extracellular matrix and cell function change with age and are considered to be the most important factors in the development and progression of osteoarthritis. The multifaceted nature of joint disease indicates that the contribution of cell death can be an important factor at early and late stages of osteoarthritis. Therefore, the pharmacologic inhibition of cell death is likely to be clinically valuable at any stage of the disease. In this article, we will discuss the close association between diverse changes in cartilage aging, how altered conditions influence chondrocyte death, and the implications of preventing cell loss to retard osteoarthritis progression and preserve tissue homeostasis.

Proteomic analysis of blastema formation in regenerating axolotl limbs

BACKGROUND

Following amputation, urodele salamander limbs reprogram somatic cells to form a blastema that self-organizes into the missing limb parts to restore the structure and function of the limb. To help understand the molecular basis of blastema formation, we used quantitative label-free liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS)-based methods to analyze changes in the proteome that occurred 1, 4 and 7 days post amputation (dpa) through the mid-tibia/fibula of axolotl hind limbs.

RESULTS

We identified 309 unique proteins with significant fold change relative to controls (0 dpa), representing 10 biological process categories: (1) signaling, (2) Ca2+ binding and translocation, (3) transcription, (4) translation, (5) cytoskeleton, (6) extracellular matrix (ECM), (7) metabolism, (8) cell protection, (9) degradation, and (10) cell cycle. In all, 43 proteins exhibited exceptionally high fold changes. Of these, the ecotropic viral integrative factor 5 (EVI5), a cell cycle-related oncoprotein that prevents cells from entering the mitotic phase of the cell cycle prematurely, was of special interest because its fold change was exceptionally high throughout blastema formation.

CONCLUSION

Our data were consistent with previous studies indicating the importance of inositol triphosphate and Ca2+ signaling in initiating the ECM and cytoskeletal remodeling characteristic of histolysis and cell dedifferentiation. In addition, the data suggested that blastema formation requires several mechanisms to avoid apoptosis, including reduced metabolism, differential regulation of proapoptotic and antiapoptotic proteins, and initiation of an unfolded protein response (UPR). Since there is virtually no mitosis during blastema formation, we propose that high levels of EVI5 function to arrest dedifferentiated cells somewhere in the G1/S/G2 phases of the cell cycle until they have accumulated under the wound epidermis and enter mitosis in response to neural and epidermal factors. Our findings indicate the general value of quantitative proteomic analysis in understanding the regeneration of complex structures.

Activation and dedifferentiation of chondrocytes: implications in cartilage injury and repair

Cartilage injury remains a major challenge in orthopedic surgery due to the fact that articular cartilage has only a limited capacity for intrinsic healing. Cartilage impaction is followed by a post-traumatic inflammatory response. Chondrocytes and synoviocytes are activated to produce inflammatory mediators and degradative enzymes which can induce a progradient cartilage self-destruction finally leading to secondary osteoarthritis (OA). However, an anti-inflammatory compensatory response is also detectable in cartilage by up-regulation of anti-inflammatory cytokines, probably a temporary attempt by chondrocytes to restore cartilage homeostasis. Matrix-assisted autologous chondrocyte transplantation (MACT) is a suitable technique for improving the rate of repair of larger articular cartilage defects. For MACT, autologous chondrocytes were isolated from a cartilage biopsy of a non-load bearing joint area. This technique requires sufficient expansion of differentiated autologous chondrocytes, which were then seeded on suitable biodegradable three-dimensional (3D) matrices to preform an extracellular cartilage matrix (ECM) before implantation into the defect. Cell expansion is accompanied by chondrocyte dedifferentiation, whereby substantial changes occur at multiple levels of chondrocyte synthetic profiles: including the ECM, cell surface receptors and cytoskeletal proteins. Since these dedifferentiated chondrocytes produce a non-specific mechanically inferior ECM, they are not suitable for MACT. 3D cultures are means of inducing and maintaining chondrocyte (re)differentiation and to preform ECM. The combination of MACT with anabolic growth factors and anti-inflammatory strategies using anti-inflammatory mediators might be useful for stabilizing the differentiated chondrocyte phenotype, to support neocartilage formation and inhibit post-traumatic cartilage inflammation and hence, the development of secondary OA.

A suppressive effect of prostaglandin E2 on the expression of SERPINE1/plasminogen activator inhibitor-1 in human articular chondrocytes: An in vitro pilot study

Prostaglandin E₂ (PGE₂) is expressed in articular joints with inflammatory arthropathy and may exert catabolic effects leading to cartilage degradation. As we observed in a preliminary experiment that PGE₂ suppressed the expression of SERPINE1/plasminogen activator inhibitor (PAI)-1 mRNA in chondrocytes, we focused on the effect of PGE₂ on PAI-1 in a panel of cultured chondrocytes obtained from osteoarthritic patients. Specifically, articular cartilage specimens were obtained from patients with osteoarthritis who underwent joint surgery. Isolated chondrocytes were cultured in vitro as a monolayer and stimulated with PGE₂. Stimulated cells and culture supernatants were analyzed using Western blotting and enzyme-linked immunosorbent assay. The results confirmed that the in vitro PGE₂ stimulation suppressed the expression of PAI-1 in the tested chondrocyte samples. The inhibitory effect was partly abrogated by an antagonist of EP4 receptor of PGE₂, but not by an EP2 antagonist. Although PGE₂ induced activations of mitogen-activated protein kinases (MAPK), blocking of the MAPK did not abrogate the suppressive effect of PGE₂, implying a distinct signaling pathway. In summary, prostaglandin is suggested to modulate the plasminogen system in chondrocytes. Further elucidation of the interaction might open a new avenue to understand the degradative process of cartilage.

Cytotoxicity of Mononuclear Cells as Induced by Peritoneal Dialysis Fluids: Insight into Mechanisms that Regulate Osmotic Stress-Related Apoptosis

Objective

High glucose content of peritoneal dialysis fluids (PDFs) has been shown to contribute to loss of peritoneal function during long-term peritoneal dialysis. However, hyperosmolality and hypertonicity of PDF are usually seen as similar stress events inducing osmotic stress-induced programmed cell death. In this study, we examined the impact of various osmotic agents on apoptosis induced by hyperosmolar PDFs, focusing on the mechanisms underlying the lethal effects of PDFs on peripheral blood mono-nuclear cells (PBMCs).

Methods

We assessed apoptosis and necrosis by annexin V–propidium iodide (PI) labeling, and caspase-3 activity by fluorescence assay. F-actin remodeling was measured using fluorescent phalloidin labeling.

Results

Hyperosmolality does not cause the cytotoxicity observed with PDF, but exposure to agents incapable of permeating cell membranes results in a significant increase in the percentage of apoptotic PBMCs by annexin V–PI labeling, which is confirmed by the increase in caspase-3 activity. Interestingly, inhibition of caspase-3 by Z-VAD-FMK did not suppress apoptosis. Extracellular hypertonicity produced polymerization of filamentous actin and cell shrinkage, which displayed similar time courses. Cell shrinkage was blocked by cytochalasin D, indicating an active role for actin cytoskeleton in hypertonicity-induced cell shrinkage. F-actin polymerization was related to an increase in intra-cellular ionic strength. Finally, we excluded a direct role for actin remodeling in osmotic stress-induced programmed cell death.

Conclusions

Exposure to osmolytes that cannot penetrate cell membranes results in a hypertonicity-induced apoptosis that cannot be blocked by the broad-spectrum caspase inhibitor Z-VAD-FMK. In addition, extracellular hypertonicity induced by impermeant solutes produces F-actin polymerization through an increase in intracellular ionic strength. The remodeling of the cytoskeleton does not modulate apoptosis but participates in cell shrinkage.

A role for actin in regulating apoptosis/programmed cell death: evidence spanning yeast, plants and animals

Achieving an understanding of how apoptosis/PCD (programmed cell death) is integrated within cellular responses to environmental and intracellular signals is a daunting task. From the sensation of a stimulus to the point of no return, a programme of cell death must engage specific pro-death components, whose effects can in turn be enhanced or repressed by downstream regulatory factors. In recent years, considerable progress has been made in our understanding of how components involved in these processes function. We now know that some of the factors involved in PCD networks have ancient origins that pre-date multicellularity and, indeed, eukaryotes themselves. A subject attracting much attention is the role that the actin cytoskeleton, itself a cellular component with ancient origins, plays in cell death regulation. Actin, a key cellular component, has an established role as a cellular sensor, with reorganization and alterations in actin dynamics being a well known consequence of signalling. A range of studies have revealed that actin also plays a key role in apoptosis/PCD regulation. Evidence implicating actin as a regulator of eukaryotic cell death has emerged from studies from the Animal, Plant and Fungal Kingdoms. Here we review recent data that provide evidence for an active, functional role for actin in determining whether PCD is triggered and executed, and discuss these findings within the context of regulation of actin dynamics.

Actin-targeting natural compounds as tools to study the role of actin cytoskeleton in signal transduction

Actin cytoskeleton controls a vast range of cellular processes such as motility, cytokinesis, differentiation, vesicle transport, phagocytosis, muscle contraction. A growing literature clearly demonstrated that actin cytoskeleton can play a regulating role in several signalling pathways. Cells tightly regulate actin dynamics through numerous specific proteins in order to rapidly and locally respond to various stimuli. An obvious approach to determine the involvement of actin cytoskeleton in signalling pathways is the use of actin-targeting natural compounds. These drugs modulate actin dynamics, accelerating either polymerization or depolymerization, through various mechanisms. This review focus on the use of these actin-targeting drugs as tools to demonstrate the role of actin cytoskeleton in several signal transduction pathways such as those initiated from antigen receptor in T and B cells or those involving mitogen-activated protein kinases (MAPKs) or transcription factors NF-kappaB and SRF (serum response factor). In this last case (SRF), the use of various actin-targeting drugs participated in the elucidation of the molecular mechanism by which actin regulates SRF-mediated transcription.

IL-1beta regulates FHL2 and other cytoskeleton-related genes in human chondrocytes

In osteoarthritis (OA), cartilage destruction is associated not only with an imbalance of anabolic and catabolic processes but also with alterations of the cytoskeletal organization in chondrocytes, although their pathogenetic origin is largely unknown so far. Therefore, we have studied possible effects of the proinflammatory cytokine IL-1beta on components of the cytoskeleton in OA chondrocytes on gene expression level. Using a whole genome array, we found that IL-1beta is involved in the regulation of many cytoskeleton-related genes. Apart from well-known cytoskeletal components, the expression and regulation of four genes coding for LIM proteins were shown. These four genes were previously undescribed in the chondrocyte context. Quantitative PCR analysis confirmed significant downregulation of Fhl1, Fhl2, Lasp1, and Pdlim1 as well as Tubb and Vim by IL-1beta. Inhibition of p38 mitogen-activated protein kinase (MAPK) by SB203580 counteracted the influence of IL-1beta on Fhl2 and Tubb expression, indicating partial involvement of this signaling pathway. Downregulation of the LIM-only protein FHL2 was confirmed additionally on the protein level. In agreement with these results, IL-1beta induced changes in the morphology of chondrocytes, the organization of the cytoskeleton, and the cellular distribution of FHL2. We conclude that L-1beta is involved in the regulation of various cytoskeletal components in human chondrocytes including the multifunctional protein FHL2. This might be relevant for the pathogenesis of OA.

Elucidation of the signaling network of COX-2 induction in sheared chondrocytes: COX-2 is induced via a Rac/MEKK1/MKK7/JNK2/c-Jun-C/EBPβ-dependent pathway

Shear stress is a pathophysiologically relevant mechanical signal in cartilage biology and tissue engineering. Cyclooxygenase-2 (COX-2) is a pivotal proinflammatory enzyme, which is induced by mechanical loading-derived shear stress in chondrocytes. In the present study, we investigated the transcriptional machinery and signaling pathway regulating shear-induced COX-2 expression in human chondrocytic cells. Deletion and mutation analyses of the human cox-2 promoter reveal that the CCAAT/enhancer-binding protein (C/EBP) and activator protein-1 (AP-1) predominantly contribute to the shear-induced cox-2 promoter activity. Supershift assays disclose that C/EBPβ, but not C/EBPα or C/EBPδ, binds to the C/EBP site, whereas c-Jun binds to AP-1. Individual gene knockdown experiments demonstrate the direct regulation of C/EBPβ expression by c-Jun, and the critical roles of both c-Jun and C/EBPβ in shear-induced COX-2 synthesis. Our studies also indicate that Rac and, to a lesser extent, Cdc42 transactivate MEKK1, which is, in turn, responsible for activation of mitogen-activated protein kinase kinase 7 (MKK7). MKK7 regulates c-Jun NH2-terminal kinase 2 activation, which, in turn, triggers the phosphorylation of c-Jun that controls shear-mediated COX-2 upregulation in chondrocytes. Reconstructing the signaling network regulating shear-induced COX-2 expression and inflammation may provide insights to optimize conditions for culturing artificial cartilage in bioreactors and for developing therapeutic interventions for arthritic disorders.

Oligomycin A and the IPLB-LdFB insect cell line: actin and mitochondrial responses

Oligomycin A, an inhibitor of mitochondrial ATP synthase, provokes simultaneous and different responses in IPLB-LdFB insect cell line. The oligomycin A treatment causes mitochondrial loss, increase in reactive oxygen species (ROS), destabilization/reorganization of the actin microfilaments and, finally, autophagic cell death. We speculate that oligomycin A affects the mitochondria and that the impairment of these organelles leads to the generation of ROS in quantities that exceed the antioxidant capacity of the cell. This in turn would lead to a feedback loop of increased mitochondrial impairment, amplification of ROS production and the removal of damaged organelles through autophagy.

One-hit stochastic decline in a mechanochemical model of cytoskeleton-induced neuron death I: Cell-fate arrival times

Much experimental evidence shows that the cytoskeleton is a downstream target and effector during cell death in numerous neurodegenerative diseases, including Parkinson's, Huntington's, and Alzheimer's diseases. However, recent evidence indicates that cytoskeletal dysfunction can also trigger neuronal death, by mechanisms as yet poorly understood. This is the first of two papers in which we study a mathematical model of cytoskeleton-induced neuron death. In our model, assembly control of the neuronal cytoskeleton interacts with both cellular stress levels and cytosolic free radical concentrations to trigger neurodegeneration. This trigger mechanism is further modulated by the presence of cell interactions in the form of a diffusible toxic factor released by dying neurons. We find that, consistent with empirical observations, our model produces one-hit exponential and sigmoid patterns of cell dropout. In all cases, cell dropout is exponential-tailed and described accurately by a gamma distribution. The transition between exponential and sigmoidal is gradual, and determined by a synergetic interaction between the magnitude of fluctuations in cytoskeleton assembly control and by the degree of cell coupling. We conclude that a single mechanism involving neuron interactions and fluctuations in cytoskeleton assembly control is compatible with the experimentally observed range of neuronal attrition kinetics.

Beta-endorphin regulation of MAPKs in cultured human articular chondrocytes: MAPK inhibitors prevent the increase of IL-1 beta protein levels during beta-endorphin stimulation

We investigated the effect of beta-endorphin on the activities of mitogen-activated protein kinases in cultured human articular chondrocytes in order to elucidate its effect on cartilage. Monolayer cultures of chondrocytes obtained from patients undergoing total knee arthroplasty were treated with 60, 600, or 6000 ng/ml beta-endorphin, or 100 ng/ml naltrexone combined with 600 ng/ml beta-endorphin. The regulation of three major mitogen-activated protein kinases phosphorylation, ERKp44/p42, p38, and JNK, was determined by Western blotting. We also examined the influence of specific mitogen-activated protein kinase inhibitors on IL-1 beta protein levels during beta-endorphin stimulation. The results demonstrate that beta-endorphin, dependent on concentration and duration of stimulation, significantly affected the activation of the three mitogen-activated protein kinases in cultured human articular chondrocytes. Naltrexone in some cases significantly regulated the mitogen-activated protein kinases in different ways when added to beta-endorphin 600 ng/ml. Furthermore, specific mitogen-activated protein kinase inhibitors hindered the increase of IL-1 beta during beta-endorphin incubation. The effect of beta-endorphin seen in this study is considered critical for the production of several mediators of cartilage damage in an arthritic joint.

Molecular signaling in necrotizing enterocolitis: regulation of intestinal COX-2 expression

Necrotizing enterocolitis (NEC) is the most common surgical emergency in premature infants. The underlying etiology of NEC remains unknown, although bacterial colonization of the gut, formula feeding, and perinatal stress have been implicated as putative risk factors. The disease is characterized by exuberant gut inflammation leading to ischemia and coagulation necrosis of the intestinal epithelium. The molecular and cellular mechanisms responsible for these pathologic changes are poorly understood. It has been shown that various exogenous and endogenous mediators such as lipopolysaccharide, inflammatory cytokines, platelet activating factor, and nitric oxide may play a role in the pathogenesis of NEC. Recent studies in our laboratory and others have established a link between NEC and activation of cyclooxygenase-2, the enzyme that catalyzes the rate-limiting step in the biosynthesis of prostanoids. The challenge is in defining the molecular signaling pathways leading to accumulation of these mediators early in the disease progression, before the onset of tissue necrosis and systemic sepsis. Identification and characterization of these pathways could lead to the development of novel treatment strategies to alleviate the morbidity and mortality associated with NEC.

Regulation of chondrocyte differentiation by actin-severing protein adseverin

The importance of actin organization in controlling the chondrocyte phenotype is well established, but little is known about the cytoskeletal components regulating chondrocyte differentiation. Previously, we have observed up-regulation of an actin-binding gelsolin-like protein in hypertrophic chondrocytes. We have now identified it as adseverin (scinderin). Adseverin is drastically up-regulated during chondrocyte maturation, as shown by Northern blot analysis, in situ hybridization, and real-time RT-PCR. Its expression is positively regulated by PKC and MEK signaling as shown by inhibitory analyses. Over-expression of adseverin in non-hypertrophic chondrocytes causes rearrangement of the actin cytoskeleton, a change in cell morphology, a dramatic (3.5-fold) increase in cell volume, and up-regulation of Indian hedgehog (Ihh) and of collagen type X--all indicative of chondrocyte differentiation. These changes are mediated by ERK1/2 and p38 kinase pathways. Thus, adseverin-induced rearrangements of the actin cytoskeleton may mediate the PKC-dependent activation of p38 and Erk1/2 signaling pathways necessary for chondrocyte hypertrophy, as evidenced by changes in cell morphology, increase in cell size and expression of the chondrocyte maturation markers. These results demonstrate that interdependence of cytoskeletal organization and chondrogenic gene expression is regulated, at least in part, by actin-binding proteins such as adseverin.

Akt signaling regulates actin organization via modulation of MMP-2 activity during chondrogenesis of chick wing limb bud mesenchymal cells

Endochondral ossification is initiated by the differentiation of mesenchymal precursor cells to chondrocytes. This process is characterized by a strong interdependence of cell shape and cytoskeletal organization accompanying the onset of chondrogenic gene expression, but the molecular mechanisms mediating these interactions are not known. In this study, we hypothesized that the activation of matrix metalloproteinase (MMP)-2 would be involved in the reorganization of the actin cytoskeleton and that this would require an Akt-dependent signaling pathway in chick wing bud mesenchymal cells. The pharmacological inhibition of Akt signaling resulted in decreased glycosaminoglycan synthesis and reduced the level of active MMP-2, leading to suppressed cortical actin organization which is characteristic of differentiated chondrocytes. In addition, the exposure of cells to bafilomycin A1 reversed these chondro-inhibitory effects induced by inhibition of Akt signaling. In conclusion, our data indicate that Akt signaling is involved in the activation of MMP-2 and that this Akt-induced activation of MMP-2 is responsible for reorganization of the actin cytoskeleton into a cortical pattern with parallel rounding of chondrogenic competent cells.

Role of RhoC in digit morphogenesis during limb development

Here we report a new role for the small GTPase RhoC in the control of limb chondrogenesis. Expression of rhoC is a precocious marker of the zeugopodial and digit blastemas and is induced by treatments with TGFbetas preceding the formation of ectopic digits. As development progresses, expression of rhoC outlines the growing distal tip of the digits, and marks the regions of interphalangeal joint formation. Functional experiments show that RhoC is a negative regulator of chondrogenesis, which controls digit outgrowth and joint segmentation. These functions appear to be mediated by reorganization of the actin cytoskeleton and modification of the adhesive properties of the mesenchymal cells.

Human chorionic gonadotropin and decidualization in vitro inhibits cytochalasin-D-induced apoptosis in cultured endometrial stromal fibroblasts

Endometrial apoptosis increases from the proliferative phase through the secretory phase and peaks at menses. However, with the onset of pregnancy, the corpus luteum is rescued and stromal cells, instead of undergoing apoptosis, reorganize the cytoskeleton and then begin to differentiate. We hypothesized that in the presence of hormones (estradiol-17beta and medroxyprogesterone acetate), chorionic gonadotropin (hCG) as an early embryonic signal, and induction of decidualization by dibutyryl-cAMP (dbcAMP), endometrial stromal cells are rescued by the regulation of proteins that inhibit apoptosis. The percentage of cells stained with annexin V, an early apoptotic marker, increased dramatically after cytoskeletal disruption with cytochalasin D compared with non-cytochalasin-D-treated controls (P < 0.05). However, treatment of cells with hCG or dbcAMP in the presence of hormones significantly (P < 0.05) decreased the percentage of annexin-V-stained cells compared with cells treated with cytochalasin D alone. This inhibition was further confirmed by immunodetection of cleaved caspase-3 and terminal deoxynucleotidyl transferase dUTP nick end labeling staining. The inhibition of apoptosis by hCG and dbcAMP was via the intrinsic pathway because the cytochalasin-D-treated cells stained intensely for Bax, whereas the cells treated with hormones, hCG, or dbcAMP stained predominantly for Bcl-2. Treatment of cytochalasin-D-treated cells with hormones and dbcAMP resulted in an increase in the secretion of IGF-binding protein-1 (IGFBP-1) and prolactin. Treatment of cytochalasin-D-treated cells with recombinant IGFBP-1 and prolactin also inhibited apoptosis. These data suggest that under in vitro conditions, both hCG and the induction of decidualization play a direct role in preventing uterine stromal cells from undergoing apoptosis. Furthermore, this inhibition of apoptosis may be mediated in part by IGFBP-1 and prolactin and the alteration in the expression of Bcl-2 and Bax.

Caspases 3 and 9 are translocated to the cytoskeleton and activated by thrombin in human platelets. Evidence for the involvement of PKC and the actin filament polymerization

Platelets express, among others, initiator caspase 9 and effector caspase 3. Upon activation by physiological agonists, calcium ionophores or under shear stress they might develop apoptotic events. Although it is well known that the cytoskeletal network plays a crucial role in apoptosis, the relationship between caspases 3 and 9 and the cytoskeleton is poorly understood. Here we demonstrate that the physiological agonist thrombin is able to induce activation of caspases 3 and 9 in human platelets and significantly increases the amount in the cytoskeleton of the active forms of both caspases and the procaspases 3 and 9. After stimulation with thrombin the amount of active caspases 3 and 9 in the cytosolic and cytoskeletal fractions were significantly reduced in Ro-31-8220-treated cells, which demonstrates that caspases activation and association with the cytoskeleton needs the contribution of PKC. Inhibition of actin polymerization by cytochalasin D inhibits translocation and activation of both caspases, suggesting that thrombin stimulates caspase 3 and 9 activation and association with the reorganizing actin cytoskeleton. Finally, our results show that inhibition of thrombin-induced caspase activation has no effect on their translocation to the cytoskeleton although impairment of thrombin-evoked caspase translocation has negative effects on caspase activity, suggesting that translocation to the cytoskeleton might be important for caspase activation by thrombin in human platelets.

Actin depolymerization is sufficient to induce programmed cell death in self-incompatible pollen

Self-incompatibility (SI) prevents inbreeding through specific recognition and rejection of incompatible pollen. In incompatible Papaver rhoeas pollen, SI triggers a Ca²⁺ signaling cascade, resulting in the inhibition of tip growth, actin depolymerization, and programmed cell death (PCD). We investigated whether actin dynamics were implicated in regulating PCD. Using the actin-stabilizing and depolymerizing drugs jasplakinolide (Jasp) and latrunculin B, we demonstrate that changes in actin filament levels or dynamics play a functional role in initiating PCD in P. rhoeas pollen, triggering a caspase-3–like activity. Significantly, SI-induced PCD in incompatible pollen was alleviated by pretreatment with Jasp. This represents the first account of a specific causal link between actin polymerization status and initiation of PCD in a plant cell and significantly advances our understanding of the mechanisms involved in SI.

Modulation of chondrocyte adhesion to collagen by echistatin

Primary chondrocytes from quail embryo epiphysis (quail epiphyseal chondrocytes, QEC) can grow either in suspension or in monolayer. In this study, the adhesion of QEC to collagen II was used as a model to study the regulation of the ligand-binding activity of integrin receptors that allows these cells to undergo a rapid transition from suspension to an adherent state. Preincubation of suspension QEC (QECSP) with the disintegrin echistatin increased by 40% their adhesion to collagen II. An inverse relationship between immobilized collagen density and echistatin-induced increase of chondrocyte adhesion was observed, thus suggesting that the disintegrin acts by increasing the ligand-binding affinity of collagen receptor(s). Further, echistatin activity does not appear to depend upon a direct binding of the disintegrin to collagen receptor(s). In fact, immobilized anti-beta1 antibodies, but not immobilized echistatin, served as effective binding sites for QECSP. Echistatin failed to stimulate chondrocyte adhesion to collagen in the presence of metabolic inhibitors, while an activating anti-beta1 antibody was still effective. Thus, echistatin may promote cell adhesion by interfering with energy-dependent signals that keep the collagen receptor(s) in a low-affinity state. Adhesion experiments performed in the presence of pharmacological inhibitors indicate that phosphatidyl inositol 3-kinase (PI3-K)/protein kinase C (PKC) and protein kinase A (PKA) pathways may transmit opposing signals on chondrocyte adhesion, and that collagen receptors are kept in a low-affinity state by PI3-kinase/PKC signalling. Since echistatin is a high-affinity ligand for alphavbeta3 integrin, the effect of the function-blocking anti-alphavbeta3 antibody LM609 was investigated. Like echistatin, LM609 stimulated chondrocyte adhesion to collagen and failed to support their attachment. Therefore, our data suggest that alphavbeta3-antagonists might regulate the binding activity of the beta1 collagen receptor, which in turn leads to the rapid transition of chondrocytes from suspension to an adherent state.

JNK and PI3K differentially regulate MMP-2 and MT1-MMP mRNA and protein in response to actin cytoskeleton reorganization in endothelial cells

Increased production and activation of matrix metalloproteinase-2 (MMP-2) are critical events in skeletal muscle angiogenesis and are known to occur in response to mechanical stresses. We hypothesized that reorganization of the actin cytoskeleton would increase endothelial cell production and activation of MMP-2 and that this increase would require a MAPK-dependent signaling pathway in endothelial cells. The pharmacological actin depolymerization agent cytochalasin D increased expression of MMP-2 and membrane type 1-matrix metalloproteinase (MT1-MMP) mRNA, and this was reduced significantly in the presence of the JNK inhibitor SP600125. Activation of JNK by anisomycin was sufficient to induce expression of both MMP-2 and MT1-MMP mRNA in quiescent cells. Downregulation of c-Jun, a downstream target of JNK, with small interference (si)RNA inhibited MMP-2 expression in response to anisomycin. Inhibition of phosphoinositide 3-kinase (PI3K), but not JNK, significantly decreased the amount of active MMP-2 following cytochalasin D stimulation with a concurrent decrease in MT1-MMP protein. Physiological reorganization of actin occurs during VEGF stimulation. VEGF-induced MMP-2 protein production and activation, as well as MT1-MMP protein production, depended on PI3K activity. VEGF-induced MMP-2 mRNA expression was reduced by inhibition of JNK or by treatment with c-Jun siRNA. In summary, our results provide novel insight into the signaling cascades initiated in the early stages of angiogenesis through the reorganization of the actin cytoskeleton and demonstrate a critical role for JNK in regulating MMP-2 and MT1-MMP mRNA expression, whereas PI3K regulates protein levels of both MMP-2 and MT1-MMP.

Lipopolysaccharide induces cyclooxygenase-2 in intestinal epithelium via a noncanonical p38 MAPK pathway

Necrotizing enterocolitis (NEC), a severe intestinal inflammation in neonates, occurs following bacterial colonization of the gut. LPS-induced production of inflammatory factors in immature enterocytes may be a factor in NEC. Previously, we described LPS-induced p38 MAPK-dependent expression of cyclooxygenase-2 (COX-2) in rat IEC-6 cells. In this study, we examine COX-2 expression in newborn rat intestinal epithelium and further characterize the mechanisms of COX-2 regulation in enterocytes. Induction of NEC by formula feeding/hypoxia increased phospho-p38 and COX-2 levels in the intestinal mucosa. Celecoxib, a selective COX-2 inhibitor, exacerbated the disease, suggesting a protective role for COX-2. COX-2 was induced in the intestinal epithelium by LPS in vivo and ex vivo. The latter response was attenuated by the p38 inhibitor SB202190, but not by inhibitors of ERK, JNK, or NF-kappaB. In IEC-6 enterocytes, COX-2 was induced by the expression of MAPK kinase 3 EE (MKK3EE), a constitutive activator of p38, but not of activators of ERK or JNK pathways. However, neither MKK3/6 nor MKK4, the known p38 upstream kinases, were activated by LPS. Dominant-negative MKK3 or MKK4 or SB202190 failed to prevent LPS-induced, p38-activating phosphorylation, ruling out important roles of these kinases or p38 autophosphorylation. LPS increased COX-2 and activating phosphorylation of p38 with similar dose-response. Blockade of LPS-induced expression of COX-2-luciferase reporter and destabilization of COX-2 message by SB202190 indicate that p38 regulates COX-2 at transcription and mRNA stability levels. Our data indicate that p38-mediated expression of COX-2 proceeds through a novel upstream pathway and support the role of the neonate's enterocytes as bacterial sensors.

Prostacyclin prevents nitric oxide-induced megakaryocyte apoptosis

1 We have previously demonstrated that nitric oxide (NO) triggers CD34(+)-derived megakaryocyte apoptosis. We here show that prostacyclin (PGI(2)) inhibits PAPA/NO-induced megakaryocyte death detected by fluorescent microscopy and flow cytometry. 2 The cAMP-specific phosphodiesterase inhibitor, Ro 20-1724, and the permeable analog dibutyryl-cAMP also delayed apoptosis. PGI(2) effect was fully prevented when adenylyl cyclase activity was suppressed by SQ 22536, and partially reversed by the permeable protein kinase A inhibitor PKI 14-22 amide. ELISA showed that while both PGI(2) and NO alone or synergistically raised cAMP, only NO was able to increase intracellular cGMP levels. 3 Treatment of megakaryocytes with PGI(2) abolished both basal and NO-raised cGMP levels. Addition of 8-pCPT-cGMP or activation of soluble guanylyl cyclase by BAY 41-2272 induced cell death in a concentration-dependent manner, and ODQ, an inhibitor of guanylyl cyclase, prevented both PAPA/NO- or BAY 41-2272-induced apoptosis. Specific cGMP phosphodiesterase inhibition by Zaprinast or suppression of adenylyl cyclase by SQ 22536 enhanced the PAPA/NO proapoptotic effect. 4 PGI(2) completely inhibited NO-mediated generation and the increased activity of the cleaved form of caspase-3. 5 In conclusion, our results demonstrate that contrary to their well-known direct and synergistic inhibitory effects on platelets, PGI(2) and NO regulate opposite megakaryocyte survival responses through a delicate balance between intracellular cyclic nucleotide levels and caspase-3 activity control.